The use of mobile batching apparatus and mobile mixing apparatus for concrete mixtures in particular is well known as is the use of mobile apparatus which have combined batching and mixing functions.
The most commonly employed apparatus for mixing and transportation of pre-mixed concrete and the like comprises a vehicle mounted agitator bowl which receives the ingredients for a concrete mix from a stationary batching plant and, by rotation of the agitator bowl, mixes the concrete mixture prior to and during transit to a discharge site.
Whilst generally effective for their intended purpose, there are a number of disadvantages associated with such mobile agitator bowl mixers. Only a relatively short time is available from the commencement of mixing to on site placement and as the use of chemical retardants can affect concrete quality, generally such use is restricted. This short mixing time thus limits the effective travelling radius of an agitator bowl mixer to a distance which safely can be travelled during the maximum mixing time of the concrete batch. This can be a particular problem in urban areas where a risk of transit delays due to traffic problems is a likelihood. Moreover, if the mobile transit mixer is involved in a traffic accident or otherwise suffers a mechanical breakdown, the entire batch of concrete can be lost and if this is allowed to set in the agitator bowl, this necessitates replacement of the bowl.
Accordingly it is necessary to locate a plurality of stationary batching plants at spaced locations in urban and suburban areas in order to provide effective delivery radii for pre-mixed concrete. Apart from being inefficient in terms of double handling of raw materials, this gives rise to substantial capital overheads and local inconvenience of increased vehicular traffic, dust and noise in the vicinity of batching plants.
As cities grow, town planning and environmental constraints will limit or even reduce the number of batch plants permitted in an urban environment. Already, town planning controls have placed restrictions on the location, days of operation and hours of operation. With growing constraints and competition in the industry, economic considerations have driven organisations towards larger batch plants and transit mixers with necessarily high capital investments and greater operational overheads.
With operational accuracies of larger batch plants more suited to larger transit mix batches, smaller capacity transit mixers are not well served. More importantly however, the capacity to serve a market for small to very small batches of premixed concrete has virtually been eliminated.
Generally speaking, the carrying capacity of larger agitator bowl mixers is usually maximised to carry a pre-mixed batch of about 6.4 cubic meters of concrete.
Where lesser quantities of concrete may be required, smaller agitator bowl mixers may be employed to deliver batches of, say, 0.40 to 2.2 cubic meters.
The main disadvantages of mobile agitator bowl mixers however is that in practice, they should discharge their entire load once on site and this must occur during a relatively short period of time. This can be grossly inconvenient to users of concrete mixes who require less than the predetermined mixer batch size and/or require discharge of small quantities of concrete over a prolonged period of time.
In the supply of ready mixed concrete to a site, it is customary to order an excess of about 10% over that calculated as required by the user as the cost and inconvenience of undersupply or undercalculation is too great. For example, if the quantity required is undersupplied by, say, 0.25 cubic meters, an extra delivery will be required. This can take up to one hour for delivery while a concrete finishing crew is not effectively employed, but as the minimum quantity supplied is normally 0.4 cubic meters this gives rise to wastage. Moreover there exists the risk of a "cold joint" formation in the pour if there is a shortfall in delivery or a delay in delivery of a subsequent batch.
As there are now penalties for disposal of excess concrete dumped on site, it is customary to add a retardant composition to any excess concrete in the agitator bowl and return the excess to the batching plant where it is dumped into a settling pond for separation and treatment of water and recovery of aggregate and cement fines for reworking. Environmental controls associated with re-treatment of unused concrete add substantially to capital costs of a batching plant.
Other problems associated with conventional agitator bowl delivery vehicles is that when rain is forecast or a delivery site is wet or boggy, such vehicles are not permitted on site due to the risk of becoming bogged and all of the attendant problems that arise therefrom and transit mixers cannot be held on stand-by on site.
In order to overcome the disadvantages associated with conventional agitator bowl mixers, there have been many proposals for mobile batching and mixing apparatus employing bulk containers for sand, aggregate, cement powder, water etc, metering devices for each constituent and a mixing and/or dispensing mechanism.
Australian Patent Application No. 27574/88 describes a mobile batching/mixing apparatus comprising a plurality of hoppers, each with a respective metering means for discharging dry ingredients onto a conveyor belt located below the hoppers. The conveyor belt delivers proportioned quantities of dry ingredients to an elevatable batch mixer in the form of a paddle mixer which is tiltable, in an elevated position, to discharge the contents thereof.
U.S. Pat. Nos. 5624577 and 4538916 describe highly sophisticated, purpose built vehicles which operate as mobile batching and mixing apparatuses to deliver variable quantities of pre-mixed concrete as required.
Australian Patents Nos. 575263 and 590101 describe a mobile batching and mixing apparatus which is located at the rear end of a vehicle having a tipping body. The apparatus, which is removably located in the tipping body to release the vehicle for other uses, comprises containers for water and cement powder and a plurality of metering devices for the various components of a concrete mix or the like. Bulk sand and aggregate is fed to respective metering devices under the influence of gravity when the tipping body is raised.
Partial mixing of the dry ingredients is achieved by simultaneous metering into the feed hopper of a mixing and discharge conveyor in the form of a screw auger located in a tubular housing. The dry ingredients and water are introduced at one end of the auger housing and mixing and discharge occurs as a continuous operation.
A particular disadvantage associated with the apparatus of Australian Patents Nos. 575263 and 590101 is that the hopper of the mixing and discharge conveyor must be centrally located below the outlets of the gravity fed metering devices. Accordingly the pivotally mounted inner end of the screw auger is lowered close to the ground surface as the vehicle body tips to feed the metering apparatus.
In order to compensate for this effective lowering of the discharge conveyor and also to provide effective mixing during the continuous mixing and discharge operation, the screw auger is of extended length which extends well above the upper portion of the vehicle body when the auger is in an upright travelling position.
A further disadvantage of this apparatus is that the drive motor for the auger screw is necessarily located at the outer or free end of the auger to avoid contact with the ground surface and consequent damage when the vehicle body is elevated to the required tipping position. The placement of the drive motor at free end of the auger shaft in turn necessitates a discharge opening in the auger body adjacent the free end thereof on an under surface.
Possibly the most serious disadvantage of all of the prior art apparatus referred to above is that none are capable of movement and/or operation in regions of restricted access to areas such as vehicular carparks, basement areas of houses or other buildings, mine tunnels and the like because of limitations on headroom, width, axle load, turning circle etc.
Another disadvantage associated with all of the prior art apparatus referred to above is that each represents a substantial capital investment with high operational overheads due to the physical size and complexity of the apparatus.